Determine how many solutions exist for the system of equations. ${x-y = -9}$ ${-x+y = 9}$
Answer: Convert both equations to slope-intercept form: ${x-y = -9}$ $x{-x} - y = -9{-x}$ $-y = -9-x$ $y = 9+x$ ${y = x+9}$ ${-x+y = 9}$ $-x{+x} + y = 9{+x}$ $y = 9+x$ ${y = x+9}$ Just by looking at both equations in slope-intercept form, what can you determine? ${y = x+9}$ ${y = x+9}$ Both equations have the same slope and the same y-intercept, which means the lines would completely overlap. ${1}$ ${2}$ ${3}$ ${4}$ ${5}$ ${6}$ ${7}$ ${8}$ ${9}$ ${\llap{-}2}$ ${\llap{-}3}$ ${\llap{-}4}$ ${\llap{-}5}$ ${\llap{-}6}$ ${\llap{-}7}$ ${\llap{-}8}$ ${\llap{-}9}$ ${1}$ ${2}$ ${3}$ ${4}$ ${5}$ ${6}$ ${7}$ ${8}$ ${9}$ ${\llap{-}2}$ ${\llap{-}3}$ ${\llap{-}4}$ ${\llap{-}5}$ ${\llap{-}6}$ ${\llap{-}7}$ ${\llap{-}8}$ ${\llap{-}9}$ Since any solution of ${x-y = -9}$ is also a solution of ${-x+y = 9}$, there are infinitely many solutions.